Optical shaft torsional displacement and speed sensing system

ABSTRACT

A rotating shaft torsional displacement and speed detector provides a light beam which impinges on one or both ends of the rotating shaft. Different combinations of beam altering materials such as reflecting and non-reflecting surfaces, wave retarding plates, and linear polarizers are attached to the front shaft end and rear shaft end. These beam altering materials change either the polarization or magnitude of the light beam in a predictable manner that allows for the detection of shaft speed and torsional displacement of the front shaft end with respect to the rear shaft end.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus for sensing the torsional displacement and speed of rotation of a shaft and, more particularly, to sensing the torsional displacement of a shaft under torque loading as an indication of the applied torque as well as the speed of rotation of the shaft.

Various devices and methods are known for quantitatively determining the torsional displacement, or twist, of a shaft under load as an indication of the applied torque. When torque is applied to a non-rotating shaft, the torsional displacement can be simply measured as a function of the relative torsional displacement, or twist, of one end of the shaft relative to the other. The measurement of torsional displacement is more complicated in dynamic situations where the shaft is rotating at relatively high speeds. Dynamic situations requiring calculation of the torsional displacement include, for example, turbo-machines, such as aircraft gas turbines and other turbo-shaft engines. In these devices, a compressor is located at the forward end of the engine and is connected through a main shaft to a turbine at the rear end of the engine. The torsional displacement of the main shaft is sensed as an incident to measuring engine torque, and the quantitative result is then available as an indication of engine power.

In traditional torque sensing systems for shafts, magnetic sensors are positioned at opposite ends of the engine main shaft and respond to respective toothed wheels secured to the shaft with each sensor providing an electrical pulse output at a pulse repetition rate that varies with the speed of rotation of the shaft. As the shaft is subjected to varying levels of torque, one end of the shaft is torsionally displaced, or twisted, relative to the other end. This causes a change in the phase relationship between the pulse trains from the magnetic sensors. Evaluation of the change in the phase relationship between the pulse trains allows an accurate determination of applied torque and the pulse repetition rate also allows an accurate determination of shaft speed.

In tubrbo-shaft engine applications, main shaft deflection and speed are determine by a monopole torque sensor of the type disclosed in commonly assigned U.S. Pat. No. 4,602,515 to Eichenlaub and U.S. Pat. No. 4,488,443 to Parkinson. In the disclosed structure two toothed wheels are positoned adjacent one another on the engine shaft with one of the wheels secured to the shaft and the other of the wheels secured to the end of a hollow reference sleeve. The opposite end of the reference sleeve is connected to the shaft so that torsional twisting of the shaft will cause a relative rotational displacement between the two wheels. A single magnetic pick-up provides an output signal representative of the relative position of the wheels.

While magnetic sensing systems have been developed to a relatively reliable state, they represent a comparatively expensive instrumentation system. Since the sensing system operates in temperature ranges that vary from ambient temperature at engine start-up to 1500° F. or more, the sensors must be designed with heat-resistant materials, and the electrical response characteristics as a function of temperature must be known to provide an accurate output for all operating temperatures. Additionally, magnetic sensing systems are susceptible to electro-magnetic interference (EMI) which can interfere with the correct output of the sensors. While EMI can be reduced with shielding, this solution adds considerable weight to the system and is particularly disadvantageous in airborne applications.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention, among others, to provide a system for sensing the torsional displacement of a shaft under dynamic loading that is more efficient, simpler, and less expensive than prior systems.

It is another object of the present invention to provide a system for sensing the torsional displacement of a shaft under dynamic conditions as an incident to measuring the applied torque.

It is still another object of the present invention to provide a system for sensing both torsional displacement of a shaft under dynamic loading as well as shaft rotation speed.

It is a further object of the present invention to provide a system for measuring the torsional displacement of a shaft that operates through a wide temperature range and is relatively immune to electromagnetic interference.

In view of these objects, and others, the present invention provides a retardation plate mounted to an axial face of the shaft and irradiated by linearly polarized light. Torsional deflection, applied torque, and rotation speed are determined, in part, by the changes in the retardation of the linearly polarized light, i.e., the zero degree polarization component and the 90 degree polarization component, as a function of torsional displacement of the shaft.

In one embodiment of the invention, a quarter-wave retardation plate and underlying mirror are mounted to an axially aligned interior end wall of a hollow shaft. An optical source, such as a light emitting or laser diode, and cooperating polarizer provide linearly polarized light that is directed to the retardation plate and mirror to irradiate the fast axis of the quarter-wave retardation plate at a predefined angle θ under no-load conditions. The light reflected from the mirror, having made two passes through the quarter-wave retardation plate, is accordingly retarded, i.e., the light includes both a zero degree and a 90 degree polarization component compared to the initial, linearly polarized light. When torsional displacement occurs under load conditions, the angle θ of the linearly polarized light beam irradiates the quarter-wave retardation plate changes and the magnitude of the polarization components of the retarded light beam also change. Comparison and use of the magnitudes of the no-load polarization components, viz., the zero degree and the 90 degree polarization component, with the polarization component values under load conditions allows calculation of the torsional displacement and shaft rotation speed.

In another embodiment of the present invention, a first axially aligned end wall of the shaft is provided with alternating reflective and non-reflective areas and is irradiated by polarized light to produce reflected light pulses. A polarizer is mounted to an axially aligned interior end wall of the shaft at an end opposite the first end and is irradiated by polarized light directed along the center of the shaft. Torsional displacement of the shaft under load conditions produces reflected light pulses when the transmission axis of the various polarizers are parallel. Comparison of the periods between pulses obtained from the front shaft end with the rear shaft end allows the calculation of both the shaft speed and torsional displacement.

The present invention advantageously provides a system for measuring the torsional displacement and speed of rotation of a shaft using lightweight optical components compared to prior systems.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow, taken in conjunction with the accompanying drawings, in which like parts are designated by like reference characters.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates the first embodiment of the present invention;

FIG. 2 illustrates a second embodiment of the system presented in FIG. 1;

FIG. 3 illustrates a third embodiment of the present invention;

FIG. 4 illustrates a fourth embodiment of the present invention;

FIG. 5 illustrates a fifth embodiment of the present invention;

FIG. 5A is a front end view of a shaft presented in FIG. 5; and

FIG. 6 illustrates a block diagram of the circuitry used to detect the torsional displacement and speed of the rotating shaft in the third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, a preferred embodiment of a shaft torsional deflection and speed sensing system in accordance with the present invention is designated generally by the reference character 10. The system 10 is designed to measure the torsional deflection and rotation speed w of a shaft 28 mounted for rotation about a longitudinal axis A_(x). The shaft 28 is defined as a hollow cylinder about the axis A_(x) and includes end walls 32 and 35 at the forward and rearward ends, respectively, of the shaft 28. The cylindrical wall surface 29 of the shaft 28 is provided with four equi-spaced windows 30 into which light is introduced and from which light exits to provide the desired torsional deflection and rotational speed information w as discussed more fully below.

A pyramidal-shaped, four-fold mirror 34 is secured to the forward end wall 32 along the axis of rotation A_(x) so that light entering from the windows 30 along a path 24 will be reflected by the corresponding inclined reflecting surface of the mirror 34 in the general direction of the axis A_(x) toward the rear end wall 35 of the shaft 28. A flat reflector, preferably in the form of a mirror 40, is secured to the rear end wall 35 with a quarter-wave retardation plate 38 secured to and overlying the surface of the mirror 34. The four-fold mirror 34 and the quarter-wave retardation plate 38 and its mirror 40 are aligned along the axis A_(x) to that light directed along the path 24 through one of the windows 30 will be reflected by the four-fold mirror 34 in the direction of and through the quarter-wave retardation plate 38. Similarly, the light reflected by the mirror 40 will be directed through the quarter-wave retardation plate 38 along light path 50 to the four-fold mirror 34 for reflection through the window 30 along the light path 48.

Light is introduced into the shaft 28 by an optical circuit which includes an optical source 12, such as a laser diode, that directs light along a path 14 through a linear polarizer 16. The light is preferably monochromatic and in the 800 nm to 1500 nm wavelength range. In an aircraft engine application, a wavelength of 800 nm is preferred, since condensate materials, known as `coke`, that accumulate on surfaces within the engine are acceptably transmissive to 800 nm energy. The linearly polarized light is directed to and enters a beam splitter 20 with a portion of the incident light transmitted through the beam splitter 20 along the light path 24 to the windows 30 of the shaft 28, another portion reflected along a path 22 to a polarizing beam splitter 54, and another portion reflected to a diffuser 25 and a reflector in the form of a mirror 26.

The linearly polarized light that is transmitted through the beam splitter 20 along the light path 24 passes through the windows 30 of the rotating shaft 28 at an angle that is perpendicular to the axis A_(x) of shaft 28. The light transmitted along light path 24 passes through one of the windows 30 and strikes one of the four reflecting surfaces of the four-fold mirror 34. The light is reflected at a right angle relative to the light path 24 and is directed toward the surface of the quarter-wave retardation plate 38 at the other end of the shaft 28. In FIG. 1, the direction of polarization of the light travelling along the axis A_(x) is represented by the vibration direction 46.

The quarter-wave retardation plate 38 is mounted to the rear end wall 35 of the shaft 28 so that its fast axis, represented at 42, is at some predetermined angle θ, when the shaft 28 is under no torsional load and in its torsionally undeflected state, with respect to the vibration direction 46 of the light after passing through one of the windows 30 and reflection by the four-fold mirror 34, that is, any angle except 0°, 90°, 180° or 270°. The incident light from the four-fold mirror 34 enters and passes through the quarter-wave retardation plate 38 and is reflected by the mirror 40 to again pass through the quarter-wave retardation plate 38 toward the four-fold mirror 34. The orientation of the quarter-wave retardation plate 38 provides reflected light from the quarter-wave retardation plate 38 that is retarded relative the incident light from the four-fold mirror 34. For example, where the incident light provided along light path 24 has a 0° polarization component only, the reflected and retarded light from the quarter-wave retardation plate 38 will have both a 0° polarization component and a 90° polarization component because of the retardation effect of the quarter-wave retardation plate 38.

In the case where the incident light provided along the light path 24 is linearly polarized and has a vibration direction 46 with only a 0° polarization component when it first enters the quarter-wave retardation plate 38, and, depending upon the initial angle θ between the vibration direction 46 and the fast axis 42 when the incident light from light path 24 strikes the quarter-wave retardation plate 38, the reflected and retarded light returned along light path 50 will be linearly polarized and have a 0° polarization component and a 90° polarization component having magnitudes that are a function of the angle θ.

For example, if the angle θ between the vibration direction 46 of the incident light from the four-fold mirror 34 and the fast axis 42 is equal to 45°, the linearly polarized and retarded light reflected along the light path 50 toward the four-fold mirror 38 will have equal 0° and 90° polarization components. The 45° angle θ results in polarization components of equal magnitude since the quarter-wave retardation plate 38 effectively functions as a half-wave retardation plate. When incident light provided along the light path 24 passes through quarter-wave retardation plate 38 twice because of the mirror 40, the extraordinary component of the incident light along light path 24 goes through a phase change of 180° before it exits the quarter-wave retardation plate 38 so that the reflected and retarded light directed to the four-fold mirror 34 will have a 0° polarization component and a 90° polarization component of equal magnitude. It should also be noted that when the incident light along the light path 24 passes through a different one of the four windows 30, its vibration direction 46 strikes the fast axis 42 at a different angle θ. However, the relative position of the vibration direction 46 of the incident light provided along the light path 24 with respect to the fast axis 42 is always an equivalent because of the equal circumferential spacing of the windows 30 on the cylindrical surface 29 of the shaft 28. Accordingly, the 0° polarization component and the 90° polarization component will have equivalent magnitudes independent of the window 30 through which the light directed along light path 24 enters.

After the incident light along light path 24 has its polarization state changed, it is reflected by the same reflecting surface of the four-fold mirror 34 that the incident light along light path 24 passes through the same window 30 along the path 48 to the beam splitter 20. After being reflected by the beam splitter 20, the retarded light beam 50 strikes the polarizing beam splitter 54 which splits the retarded light beam 50 into a 90° polarization component beam along light path 56 and a 0° polarization component beam along light path 60. The 90° polarization component beam 56 is detected by a 90° polarization component detector 58, and, in a similar manner, the 0° polarization component beam 60 is detected by a 0° polarization component detector 62. The resulting signals are provided to and processed by a signal processor 64, the function of which is described more fully below.

When the shaft 28 is in its no-load, torsionally undeflected state, the signal processor 64 senses and stores the initial values of the 90° polarization component and the 0° polarization component for use in computing the torsional deflection using the corresponding values when the shaft 28 is in a loaded, torsionally deflected state. In addition, values for the 0° and 90° polarization components for all angles θ between the vibration direction 46 and the fast axis 42 are likewise determined to correlate sensed 0° and 90° polarization component values with the initial value to provide a torsional deflection signal. While rotation of the shaft 28 causes the linearly polarized light along the light path 24 to strike the quarter-wave retardation plate 38 four times per revolution, the vibration direction 46 of the incident light from light path 24 will strike the fast axis 42 at the same relative angle θ every time for any given torsional deflection.

When the shaft 28 is subjected to a torsional load, the forward and rearward ends will be rotationally displaced relative to one another to cause a corresponding relative rotation of the quarter-wave retardation plate 38, a change in the orientation of the fast axis 42, and a change in the angle θ between the fast axis 42 and the vibration direction 46. Because of these differences, the values of the 0° polarization component and the 90° polarization component of the retarded light returned toward the four-fold mirror 34 from the quarter-wave retardation plate 38 will also vary from the polarization component values under the no-load, torsionally undeflected conditions.

The retarded light is reflected by the four-fold mirror 34 to the light path 48 and presented to the beam splitter 20 and the polarization splitter 54 and split into a 0° polarization component along light path 56 and a 90° polarization component along light path 60. The component along light oaths 56 and 60 are then detected by the detectors 58 and 60, respectively, and the detected signals sent to the signal processor 64.

The signal processor 64 uses the signal values to calculate the result of the difference of these values divided by their sum, compare this result with the predetermined values for all angles θ to determine the detected angle θ, and provide an output representative of the angular difference δθ between the detected angle θ and the angle θ sensed under the no-load, torsionally undeflected conditions. This angular difference, δθ, is the torsional displacement between the front shaft end 32 and its rear shaft end 35. The rotational shaft speed is determined by counting the frequency of the pulses detected by either the detector 58 or 60 per unit time.

As an alternative to calculating each of the values of the 0° polarization component and the 90° polarization component and storing these values in signal processor 64, Mueller calculus can be used to predict the torsional displacement for detected polarization component values. The optical power P of the detected signal follows the equation:

    P=K[1+cos 4(wt+δθ)].sup.2                      Eq. 1

where

K is a scaling factor,

w is the shaft rotation rate,

θ is the torsional displacement, and

δθ is the change in torsional displacement.

Since the shaft speed w can be determined from the the pulses per unit time from the detectors 58 and 62 and the scaling factor K and the optical power P are known, the torsional displacement θ can be derived.

The embodiment of FIG. 1 uses a quarter-wave retardation plate 38 with a mirror 40 because the configuration allows very accurate measurements to be obtained. The calculation of the difference of the 90° polarization component and the 0° polarization component divided by the sum of these components compensates for fluctuations in the intensity of the light provided by the optical source 12 and provides accurate and repeatable results. If desired, the quarter-wave retardation plate 38 can be replaced with a different type of the retardation plate that varies the phase of the extraordinary ray of the incident light by a different amount The magnitudes of the 0° polarization component and the 90° polarization component of the resulting retardation of the light under no-load conditions can still be predetermined for any angle θ for the retardation plate used. These values can then be compared to the values actually detected by the 0° polarization component detector 58 and the 90° polarization component detector 62 to determine the torsional angular displacement of the shaft 28, although the retarded light provided by the retardation plate may be linearly, elliptically, or circularly polarized. Also, the number of windows 30 provided in the shaft 28 is arbitrary, although a greater number of windows 30 increases the accuracy of the resulting measurement.

A variation of the system of FIG. 1 is shown in FIG. 2 and designated therein by reference character 10A with like parts designated by like reference characters. A half-wave retardation plate 39 at the end wall 35 is utilized in a transmission configuration by which the light reflected from the four-fold mirror 34 is directed through the half-wave retardation plate 39 with the retarded light transmitted through the half-wave retardation plate 39 along a light path 50' to the polarization beam splitter 54 for analysis as described above. As can be appreciated, the light transmitted through the half-wave retardation plate 39 includes the 0° polarization component and the 90° polarization component. After detection, the signal outputs of the detectors 58 and 62 are processed in the same way by the signal processor 64 to determine the speed of the shaft 28 and its torsional displacement.

Another embodiment of the present invention is illustrated in FIG. 3 and is designated generally therein by the reference character 10B and in which like reference characters are used for like parts. In the embodiment of FIG. 3, the windows 30 and the four-fold mirror 34 are not utilized. As shown, the forward end wall 32 of the shaft 28 is provided with a slit system 76, including orthogonally crossed slits 78 (represented in dotted line). The slits 78 serve the same purpose as the equally spaced windows 30 of the embodiments of FIGS. 2 and 3 in allowing the incident light along the light path 24 to strike the quarter-wave retardation plate 38 at the same relative angle θ. It is noted that the light path is parallel to but offset from the axis of rotation A_(x) of the shaft 28.

A further embodiment of the present invention is illustrated in FIG. 4 and designated by the reference character 10C. As shown, the shaft 28 includes two different sets of diametrically opposed windows 68 and 72. A linear polarizer 66 and the mirror 40 are attached to the rear end wall 35 of the shaft 28 rather than the quarter-wave retardation plate 38 and mirror 40 of FIG. 1. The linear polarizer 66 is mounted on the rear shaft end 35 so that when the incident light along light path 24 travels through either of the two windows 68, the vibration direction 46 of the incident light is parallel to the transmission axis 70 of the linear polarizer 66. However, when the shaft 28 is rotated 90° and the incident light passes through the windows 72, the vibration direction 46 is perpendicular to the transmission axis 70. When the shaft 28 is torsionally unloaded, the resulting light along light path 74 will have its largest magnitude when the incident light along light path 24 travels through the windows 68 and will have a minimum magnitude, when the incident light from light path 24 travels through the windows 72. Measuring the magnitude of the resulting light along path 74 for all angles θ between the transmission axis 70 and the vibration direction 46 allows for the determination of the torque load on the shaft 28 since the torsional displacement causes a different detected magnitude of the light intensity.

Another embodiment of the present invention is illustrated in FIGS. 5 and 6 and is designated generally by the reference character 10D with like parts designated by like reference characters. In the embodiment of FIG. 10D, optical sensor heads, described below, direct light into and receive light from the shaft 28. In addition, the shaft 28 is provided with a linear polarizer 66 and underlying mirror 40 on the rear end wall 35 with the forward end wall of the shaft 28 defined, as shown in FIG. 5A, by a light chopper 110 in the form of an annular rim having diametrically opposed non-reflective, i.e., light-absorbing, sectors alternated with reflective sectors 114.

As shown, a light source 80 generates and introduces light into the core of an optical fiber 84 and, in a similar manner, a light source 82 generates and introduces light into the core of the optical fiber 86. The optical sources 80 and 82 can take the form of light emitting or laser diodes. The light from the optical fibers 84 and 86 is transferred to passive, bi-directional optical couplers 88 and 90, respectively, which provide a 50/50 split ratio. The couplers 88 and 90 each have three ports arranged so that 50% of the incoming light introduced through one port is transmitted to each of the remaining two ports. A portion of the light generated from the light source 80 passes through the coupler 88 into the optical fiber 91 and travels to a sensor head 94, and, in a similar manner, a portion of the light from the light source 82 passes through the coupler 90 to the optical fiber 92 to the sensor head 96.

Both sensor heads 94 and 96 are similar and include a bulkhead connection 98, an optical fiber stub 100, and a lens 102. Each lens 102 is designed with a focal length that focuses the return light back into its respective optical fiber stub 100. The sensor head 94 also contains a linear polarizer 104 having a center bandwidth that matches the center bandwidth of the light emitted from the light source 80. In addition, the diameter of the polarizer 104 matches the diameter of the lens 102 in the sensor head 94.

The polarized light emitted from the polarizer 104 is transmitted along light path 106 and passes through the interior of the shaft 28 in a direction parallel to the longitudinal axis A_(x) of shaft 28. The polarized light irradiates and passes through the polarizer 66 and is returned along light path 107 by the mirror 40 as twice-polarized light that passes through the polarizer 104, is focused by the lens 102 into the optical fiber stub 100, and passes through the bulkhead connection 98 and the optical fiber 91 into the coupler 88. A portion of the twice-polarized return light then travels through the optical fiber 118 and is detected by the detector 120 which provides an output along circuit path 122 to the signal processor 130.

The sensor head 96 directs light along light path 108 toward the forward end of the shaft 28 (FIG. 5A) and irradiates the two non-reflective areas 112 and the two alternating reflective areas 114, the non-reflective and reflective sectors subtending the same arc. When the light from the sensor head 96 along light path 108 irradiates the reflective areas 114, a portion is reflected back along light path 109 toward the sensor head 96, is focused by the lens 102 into the optical fiber stub 100, and then passes through the bulkhead connection 98 and the optical fiber 92 into the coupler 90. A portion of the reflected light passes through the optical fiber 124 and is detected by the detector 126 which provides an output along circuit path 122 to the signal processor 130.

The detectors 120 and 126 convert the detected optical signals from the twice-polarized return light along light path 107 and the reflected light along light path 109 into electrical signals 122 and 128, respectively. Both electrical signals 122 and 128 are sent to the signal processor 130 so that the shaft speed and torsional displacement of shaft 28 can be determined, as described below in relationship to FIG. 6.

The polarizer 66 is mounted on the rear end wall 35 of the shaft 28 so that its transmission axis 70 is aligned with the transition between the reflective areas 114 and the non-reflective areas 112. This orientation allows for the detection of twice-polarized light returned along light path 107 during those times that light along light path 108 is being absorbed by the non-reflective area 112 of the chopper 110. When light is reflected along light path 109 by the reflective area 114 and is detected, the transmission axis 103 of the polarizer 104 is perpendicular to the transmission axis 70 of the polarizer 66. Accordingly, the twice-polarized light returned along light path 107, having a magnitude of zero, is extinguished to result in two pulses per revolution per channel occurring at equally spaced intervals when the shaft 28 is rotating at a constant speed.

The speed of rotation of the forward portion of the shaft 28 can be determined by measuring the time between the pulses sensed by the detector 126. Similarly, the speed of rotation of the rearward portion of the shaft 28 can be determined using the time between pulses detected by the detector 120. The torsional displacement of the forward portion of the shaft 28 with respect to the rearward portion can be determined by detecting the time difference between the pulses detected by the detector 120 and the detector 126. Dividing the time between these pulses by the period of rotation and then multiplying by a scaling factor yields the torsional displacement of the front and rear portions of he shaft 28.

The signal processor 130 is shown in circuit diagram form in FIG. 6, and, as shown, electrical signals provided on circuit paths 122 and 128, which correspond to the signals detected by the detectors 120 and 126, respectively, are each sent through respective amplifiers 132 and 132' into respective zero-crossing detectors 138 and 138'. The zero-crossing detectors 138 and 138' transform the analog signals along circuit paths 134 and 136 into digital signals along circuit paths 140 and 142 that are sent to respective pulse shapers 144 and 144'. Each pulse shaper 144 and 144' produces a short pulse on the rising or leading edge of the respective digital signal from the zero-crossing detectors 138 and 138'. Each pulse shaper 144 and 144' outputs a pulse with a fixed duration sufficiently short so that a pulse on circuit path 146, obtained from the digital signal on circuit path 140, does not overlap a pulse on circuit path 148, obtained from the digital signal on the circuit path 142. The pulse shapers 144 and 144' can also be designed to provide a pulse on both the rising and falling edges of the digital signals on circuit paths 140 and 142 to double the number of measurements that can be obtained in a single revolution of the shaft 28. The pulse on circuit path 146 is sent to one input of an OR gate 150 and the set input of a RS flip-flop 152. The pulse on circuit 148 path is sent to another input of the OR gate 150 and the reset input of the RS flip-flop 152.

Whenever the OR gate 150 detects an input pulse, an output pulse is provided along circuit path 151 to a counter 154, which receives its timing signals from an oscillator 156, and determines the time that the pulse was sent by the OR gate 150. A computer 158 reads and stores this time, determines the time duration between successive pulses, and uses these times to compute the shaft speed with the torsional displacement of the shaft 28 presented on a display 160.

If a distinction between the speed of the forward end of the shaft 28 and rearward end is desired, the output waveform along circuit path 153 from the RS flip-flop 152 can be sent to the computer 158 to indicate whether the computer 158 is reading a pulse originating from the detector 120 or from the detector 126. Furthermore, overspeed and underspeed conditions of the shaft 28 can be determined by comparing the detected shaft speed with predefined upper and lower shaft speed limits.

FIG. 6 also includes the analog equivalent of the calculation circuitry which normally is not included with the digital equivalent described above. The analog circuitry requires the use of the R-S flip-flop 152 and low-pass filters 162 and 166. Pulses along circuit paths 146 and 148, which are provided to the set and reset inputs of the R-S flip-flop 152, generate an output waveform along circuit path 153 that is proportional to any phase changes between the pulses along circuit paths 146 and 148. The output waveform along circuit path 153 is then sent to the low-pass filter 162 to convert the phase information of the output waveform along circuit path 153 into an appropriate voltage which is sent to a torsional displacement indicator 164 to display the torsional displacement. The pulse along circuit path 148 is also sent to a low-pass filter 166 to produce a voltage which is provided along a circuit path to a speed indicator 168 to display the speed of shaft 28. The signal processor 130 can also derive the torsional displacement using Mueller calculus rather than storing the magnitude of each detected signal for different angles θ. The optical power P of the detected signal follows the equation:

    P=K[1+sin 2(wt+δθ)].sup.2                      Eq. 2

when

P is the optical power,

K is a scaling factor,

w is the shaft rotation rate,

θ is the torsional displacement, and

δθ is the change in torsional displacement.

The signal processor 130 can solve this equation to determine the torsional displacement δθ. A modification of this embodiment places either a polarizer 66 or a retardation plate at both the forward end of the shaft 28 and the rearward end. The pulses are obtained in the same way and the difference between the time of pulses is determined using similar types of detectors.

The present invention advantageously provides a system for measuring the torsional displacement and speed of rotation of a shaft using lightweight optical components compared to prior systems.

Thus it will be appreciated from the above that as a result of the present invention, a shaft torsional displacement and speed sensing system is provided by which the principal objectives, among others, are completely fulfilled. It will be equally apparent and is contemplated that modification and/or changes may be made in the illustrated embodiment without departure from the invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawings are illustrative of preferred embodiments only, not limiting, and that the true spirit and scope of the present invention will be determined by reference to the appended claims and their legal equivalent. 

What is claimed is:
 1. An apparatus for determining the torsional displacement of a hollow rotating shaft having a rearward end comprising:polarization changing means, said polarization changing means comprising a retardation plate disposed on a hollow rotatably mounted shaft for changing the polarization state of light energy; light production means for producing polarized light energy and directing the light energy toward said retardation plate; detector means for detecting the change in the polarization state of the light energy caused by said polarization changing means; and means for determining the torsional displacement of the shaft from the detected change in polarization state of the light energy.
 2. An apparatus according to claim 1 wherein said second-mentioned means comprises:means for directing the light energy perpendicular to the longitudinal axis of the shaft at a forward end of the shaft, the shaft having a plurality of equally spaced windows in a surface thereof; and means for reflecting disposed on the forward end of the shaft for reflecting the light energy toward said rearward shaft end along the longitudinal axis of the shaft.
 3. An apparatus according to claim 2 wherein the light energy directing means further comprises a mirror disposed between said retardation plate and said rear shaft end.
 4. An apparatus according to claim 1 wherein said retardation plate is a half-wave retardation plate.
 5. An apparatus according to claim 1 wherein retardation plate is a quarter-wave retardation plate.
 6. An apparatus according to claim 1 wherein said means for determining said torsional displacement uses Mueller calculus to determine said torsional displacement.
 7. An apparatus for determining the torsional displacement of a hollow rotating shaft having a rearward end comprising:light production means for producing light energy having a predetermined intensity; light directing means for directing the light energy toward a hollow rotatably mounted shaft having a rearward end; a polarizer disposed on said rearward end of said shaft for changing the polarization state of the light energy; said light directing means being adapted to direct the light energy toward said polarizer disposed on said rearward end; detector means for detecting changes in the magnitudes of the polarization components of the light energy passing through said polarizer as said shaft is torsionally displaced; and means for determining the torsional displacement of said shaft from said detected changed magnitudes of the polarization components of the light energy.
 8. An apparatus according to claim 7 wherein the produced light energy is polarized.
 9. An apparatus for determining torsional displacement of a rotating shaft comprising:a rotatable hollow shaft having a forward end and a rearward end; means for producing light energy and irradiating said front end; means for producing polarized light energy and irradiating said rearward end; chopper means disposed on said front end for alternately reflecting and absorbing said produced light energy; means disposed on said rearward end for changing the optical characteristic of said polarized light; means for detecting the light energy is reflected by said chopper means; means for detecting the changed optical characteristic of the polarized light; means for determining the shaft torsional displacement using the detected changes and reflections.
 10. An apparatus according to claim 9 further comprising a mirror disposed between said polarizer and said rearward end.
 11. An apparatus according to claim 9 wherein said means for detecting said reflected light energy and said means for detecting the changed polarized light energy each effect said detections alternately.
 12. An apparatus according to claim 9 wherein said changed optical characteristic of said polarized light energy is the polarization.
 13. An apparatus according to claim 9 wherein said changed optical characteristic of said polarized light energy is the light intensity. 